Generator Sizing Calculator

About this calculator

Sizing a generator correctly requires more than adding up the wattage of everything you plan to run, because motors — pumps, compressors, air conditioners, refrigerators — draw a brief starting (surge) current that can be 2–3 times their running wattage when they switch on. The generator must be sized for total running watts + (the largest motor's starting surge − its own running watts) + a safety margin, since only one motor typically starts at a time while the others are already running.

Homeowners buying a backup or portable generator, and RV or job-site users, run this calculation before purchase because undersizing causes the generator to stall or trip its breaker the moment a compressor kicks on, even if the running-watts total looked comfortably within the generator's rated capacity — the starting surge, not the steady load, is usually what determines the minimum generator size.

This calculator takes your total running watts, the largest motor's starting surge and a safety margin percentage and returns the minimum generator kW you need, so you can shop with a number that accounts for real-world startup spikes, not just nameplate wattage.